1. Field of the Invention
This invention relates to the disinfection of water and to the control of biofouling in recirculating water systems such as cooling towers, evaporative condensers, air washers, swimming pools, hot tubs, and spas.
The invention more especially concerns methods and compositions for controlling biofouling and microorganism population levels in such systems wherein water soluble hypochlorite donors and bromide ion donors are added to the systems so as to improve biocidal effectiveness with reduced costs.
As used herein, the term "hypochlorite donor" means any compound that will generate hypochlorite species when dissolved in water.
The term "bromide ion donor" means any compound that will generate bromide ions when dissolved in water.
The term "available halogen" means the standard form for expressing the strengths or capacities of halogenating chemicals as well as for the doses in which they are applied and for the hypohalite species (HOCl, OCl.sup.--, HOBr, OBr.sup.--) which remain in the water.
The term "available chlorine" means the same as "available halogen", but refers specifically to chlorine compounds.
The term "available bromine" means the same as "available halogen", but refers specifically to bromine compounds.
The term "hypohalite species" means hypochlorous acid, hypochlorite ion, hypobromous acid and hypobromite ion.
The term "hypochlorite species" means hypochlorous acid and hypochlorite ion.
The term "hypobromite species" means hypobromous acid and hypobromite ion.
The term "bromine species" means hypobromous acid, hypobromite ion, and bromide ion.
The terms "free halogen" and "free available halogen" are used interchangeably and are defined as the concentration of halogen existing in the water as hypohalous acid, HOX, and hypohalite ion, OX.sup.--, where X is Cl or Br.
The terms "free chlorine" and "free available chlorine" are used interchangeably and are defined as the concentration of chlorine existing in the water as hypochlorous acid, HOCl, and hypochlorite ion, OCl.sup.-.
The terms "free bromine" and "free available bromine" are used interchangeably and are defined as the concentration of bromine existing in the water as hypobromous acid, HOBr, and hypobromite ion, OBr.sup.--.
The terms "combined halogen" and "combined available halogen" are used interchangeably and are defined as the concentration of halogen existing in the water in chemical combination with ammonia or organic nitrogen compounds.
The terms "combined chlorine" and "combined available chlorine" are used interchangeably and are defined as the concentration of chlorine existing in the water in chemical combination with ammonia or organic nitrogen compounds.
The terms "combined bromine" and "combined available bromine" are used interchangeably and are defined as the concentration of bromine existing in the water in chemical combination with ammonia or organic nitrogen compounds.
The terms "total halogen" and "total available halogen" are used interchangeably and are defined as the sum of "free halogen" (or "free available halogen") and "combined halogen" (or "combined available halogen").
The terms "total chlorine" and "total available chlorine" are used interchangeably and mean the same as "total halogen" and "total available halogen" but specifically refer to chlorine.
The terms "total bromine" or "total available bromine" are used interchangeably and mean the same as "total halogen" and "total available halogen" but specifically refer to bromine.
The symbol "FAvC" represents "free chlorine" and "free available chlorine" concentrations in the water.
The symbol "AvC" represents the available chlorine content of the hypochlorite donor.
The term "halogen demand" is defined as the amount of halogen which must be added to the water over a specific period of time to maintain the "free halogen" and/or "free available halogen" at a specific concentration in the water.
The term "chlorine demand" means the same as the "halogen demand" but specifically refers to "free chlorine" and/or "free available chlorine" concentrations.
The term "chlorinated isocyanuric acid derivative" means chlorinated isocyanuric acid including dichlorinated and trichlorinated isocyanuric acid, alkali metal and alkaline earth metal salts of chlorinated isocyanuric acid, and hydrates, complexes and mixtures thereof.
The term "hydantoin derivative" means an unsubstituted, halogenated (i.e. chlorinated or brominated), or alkylated hydantoin.
The term "sulfamic acid derivative" means unsubstituted, halogenated, or alkylated sulfamic acid.
The term "sulfonamide derivative" means halogenated, alkylated, or arylated sulfonamide.
The term "glycoluril derivative" means unsubstituted, halogenated, or alkylated glycoluril.
The term "succinimide derivative" means unsubstituted, halogenated, or alkylated succinimide.
The term "oxazolidinone derivative" means an unsubstituted, halogenated, alkylated, or arylated oxazolidinone.
The term "imidazolidinone derivative" means an unsubstituted, halogenated, alkylated, or arylated imidazolidinone.
The term "halogen concentration (free chlorine basis or free available chlorine basis)" means the halogen concentration in terms of free available chlorine, regardless of whether the halogen species are hypochlorite, hypobromite or mixtures thereof.
2. Related Art
Cooling towers are used to provide cooling for the air conditioning systems of office buildings, hotels and hospitals and to provide cooling for industrial processes. The water in these towers is subject to contamination from the air blown through the tower and from the fresh water used to compensate for evaporative losses and blowdown. The contamination consists of both inorganic and organic debris as well as live microorganisms capable of growing and multiplying if suitable conditions are provided. Formation of microbial deposits, known as biofouling, can occur on almost any surface exposed to an aqueous environment, causing substantial energy losses due to increased heat transfer resistance. For this and other reasons, cooling towers are adversely affected by microorganisms, e.g. bacteria, fungi, molds, and algae, by either sheer numbers of organisms, metabolic waste products generated, health hazards presented, or deposits created. Unfortunately, cooling towers provide many of the conditions ideal for microbial growth, namely favorable temperatures and moisture levels, and favorable concentrations of air and nutrients.
Air washers are used to cool, cleanse, and humidify the air in office buildings, factories, shopping malls, and the like. Due to the large amount of air drawn through the water, the growth of microorganisms is again a problem. Since the air is used directly for inhabited areas, the toxicity and odor of any compounds used for treatment of the water in the air washers must be extremely low.
Similarly, water in swimming pools, hot tubs and spas must be sanitized in order to control disease spreading microorganisms. As with air washers, the toxicity and odor of compounds used to treat the water must be extremely low.
It is customary to treat biologically contaminated water with one or more biocides to control the population of microorganisms in the water, to prevent fouling of heat exchanger surfaces, and to prevent the spread of disease. The biocides most commonly used to disinfect and sanitize water in recirculating water systems are chemicals that generate hypochlorite species when dissolved in water. There are many hypochlorite generating chemicals, but the more common ones are chlorine gas, alkali metal hypochlorites such as sodium hypochlorite, alkaline earth metal hypochlorites such as calcium hypochlorite, chlorinated hydantoins, and chlorinated isocyanuric acid derivatives.
Dry sources of biocide are often preferable to gaseous or even liquid forms because the dry forms are often safer to handle, more convenient to store and use, and more stable in storage. Moreover, one or more dry products may conveniently be fed to a recirculating water system using an erosion feeder in which water is passed through a bed of solid biocide to slowly dissolve the biocide and is then added to the recirculating water. One such erosion feeder is described in U.S. Pat. No. 3,412,021.
The different forms of hypochlorite donors all work by generating hypochlorous acid (HOCl) in solution, which provides the significant biocidal action. Hypochlorous acid has strong biocidal properties under the proper conditions. Its killing power is adversely affected, however, by alkaline pH levels and by the presence of ammonia or other nitrogenous material.
The pH of cooling water is typically regulated in the range of 8.0 to 9.0 for alkaline corrosion protection. At pH levels above 7.5, chlorine-based biocides become less effective because of the equilibrium shift from hypochlorous acid to hypochlorite ion. ##EQU1## The hypochlorite ion cannot easily penetrate microorganism cell membranes, while the uncharged hypochlorous acid can passively diffuse into cells to cause damage.
Water in recirculating water systems is also frequently contaminated with ammonia due to the decomposition of nitrogenous impurities in the water or to the leakage of ammonia from refrigeration units into the cooling water. Ammonia or chloramines are also commonly introduced into the recirculating water system by the makeup water. Hypochlorite species react with ammonia to form chloramines. Since chlorine is bound very strongly by nitrogen, the chlorine is not readily released by chloramines to the water as hypochlorite species, and the biocidal activity of the chlorine-based biocide is, therefore, greatly reduced. The fact that the chloramines are relatively stable chlorine compounds also makes it more difficult for some cooling tower systems to comply with the EPA total halogen (free halogen+combined halogen) discharge limit of 0.2 ppm. In some cases, these cooling tower systems frequently have to dechlorinate the discharge water in order to be in compliance. Moreover, chloramines have a disagreeable and irritating odor. They can be converted to odorless nitrogen gas by maintaining the appropriate free chlorine concentration in the recirculating water, but some chloramines are still volatilized into the air. Even though the amounts are negligible, chloramine odors are still noticeable. Chloramine odor is an important issue with indoor pools and spas because the air containing the volatilized chloramines is retained in the buildings long enough for the chloramine concentration to accumulate to levels that are objectionable to the consumer. Thus, the formation of chloramines in recirculating water can present a serious obstacle to the use of chlorine-based biocides.
Hypobromous acid (HOBr), which can be generated from a number of compounds including liquid bromine and N-bromo organic compounds or by reacting a bromide salt with a solution of hypochlorous acid or other oxidizing agents, is a more effective biocide on a molar basis than hypochlorous acid. Under some conditions, this superiority is quite dramatic. In particular, hypobromous acid is known to react with ammonia to produce bromamines. Bromamines, unlike chloramines, have very good biocidal activity and have a more acceptable odor. Bromamines also have a distinct advantage over chloramines because they dissipate more readily, thereby making it easier to operate cooling towers in compliance with the EPA limits for total halogen. In addition, hypobromite species are more effective than hypochlorite species at pH values above 7.5 due to the higher pK value for the equilibrium shift from hypobromous acid to hypobromite ion. ##EQU2##
In most cases where hypobromous acid is used as a biocidal agent, the hypobromous acid generating composition contains a large weight percentage of bromine. Liquid bromine, for example, is 100% bromine by weight and 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH) is 32.8% bromine by weight. This practice leads to higher costs for the bromine-based biocides since the cost of bromine is about three times the cost of chlorine per pound. Since 2.25 pounds of bromine contain the same number of moles of available halogen as only 1.0 pound of chlorine, bromine is over seven times more expensive than chlorine on a per mole basis. Even though hypobromous acid is generally superior to hypochlorous acid, the higher cost of bromine has limited the use of bromine-based biocides.
Nevertheless, in the past few years several products have been introduced into the cooling tower marketplace which take advantage of the bromine chemistry. In 1982, Nalco introduced a bromine-based product (tradename Actibrom) for use in large scale cooling towers. These towers already had chlorinators injecting gaseous chlorine for disinfection. Actibrom is simply an aqueous solution of sodium bromide, and is typically added in proportion to the chlorine gas using a separate feeder. See U.S. Pat. No. 4,451,376. Another bromine-based biocide, 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH) was introduced into the cooling tower marketplace by Great Lakes Chemical. See U.S. Pat. No. 4,297,224.
Bromine sanitizers have also gained some measure of popularity for indoor pool and spa applications, because the odor of the bromamines, formed by reaction of hypobromite species with nitrogenous wastes, is less objectionable to the consumer. Bromine sanitizers, however, have not been popular for outdoor pools because the hypobromite species are rapidly dissipated in sunlight and the sanitizer costs are considerably higher than chlorine sanitizers with cyanuric acid.
Potassium monopersulfate and sodium bromide have been marketed together as a bromine sanitizer system for spa applications. The recommended practice is to dose the spa water with sodium bromide (usually as a solution) and then add the recommended dosages of potassium monopersulfate as needed. Hypobromous acid is generated by oxidation of the bromide ion with persulfate ions as shown by the following equation: EQU KHSO.sub.5.KHSO.sub.4 +NaBr.fwdarw.HOBr+KHSO.sub.4 +NaKSO.sub.4 ( 3)
Currently available dry sources of hypobromous acid suffer from a number of disadvantages in addition to their higher cost. The hydantoin products such as BCDMH, 1,3-dichloro-5,5-dimethylhydantoin (CCDMH), 1,3-dibromo-5,5-dimethylhydantoin (BBDMH), 1,3-dichloro-5-ethyl-5-methylhydantoin (CCEMH), and 1-bromo-3-chloro-5-ethyl-5-methylhydantoin (BCEMH) have very low dissolution rates which necessitates the use of large feeder systems and high water flow rates. Moreover, in some cases it is desirable to add a large amount of available halogen at one time to rapidly clean up a recirculating water system. This is known as a "shock treatment". Such a treatment would be desired whenever a system has experienced a large amount of contamination or when microorganism growth has gotten out of control. However, the hydantoin products are generally unsuited for this application due to their low dissolution rates.
In addition, the hydantoin products are not as effective biocides as might be expected based on the amount of hypobromous acid formed, because these products also release large amounts of 5,5-dimethylhydantoin (DMH) or 5-ethyl-5-methylhydantoin (EMH) into the water, eventually leading to the buildup of high concentrations of DMH or EMH in the water. High concentrations of DMH or EMH inhibit the biocidal activity of the hypobromous acid by virtue of the following equilibria: EQU HOBr+DMH.revreaction.H.sub.2 O+BDMH (4) EQU HOBr+EMH.revreaction.H.sub.2 O+BEMH (5)
where BDMH is bromo-DMH and BEMH is bromo-EMH. This effect has previously been noted in U.S. Pat. No. 4,698,165.
As an alternative to the hydantoins, hypobromous acid may be prepared by reacting a bromide salt with a source of hypochlorite species according to the following equation: EQU HOCl+Br.sup.-- .fwdarw.HOBr+Cl (6a) EQU OCl+Br.sup.-- .fwdarw.OBr.sup.-- +Cl.sup.-- ( 6b)
as previously taught, for example, in British Patent 1,327,531 and U.S. Pat. Nos. 2,815,311; 3,975,271; and 4,119,535. The hypobromous acid formed by the above equation is the active biocide. However, in the process of killing microorganisms or oxidizing organic material, the hypobromous acid is reduced to form bromide ion, as shown by the following equation: EQU HOBr+microorganisms.fwdarw.dead microorganisms+Br.sup.-- +H.sub.2 O (7)
Thus, the bromide ion can be reused to generate more hypobromous acid by reaction with hypochlorite species as shown above in equations 6a and 6b. Because the bromide ion is continuously reused, only small amounts of bromide ion are necessary to make a chlorine-based biocide in combination with bromide salts perform as a bromine biocide.
Some prior art teaches that, when using mixtures of chlorine-based biocides in combination with bromide salts, large excesses of bromide ion should be maintained in the recirculating water. For example, British Patent No. 1,327,531 describes a process for sanitizing swimming pool water wherein the concentration of bromide is maintained at 20 to 50 mg per liter (expressed as sodium bromide) and the concentration of the hypobromite species is maintained at 0.4 mg/L. Other prior art, e.g., U.S. Pat. No. 3,975,271, suggests that when hypobromous acid is generated by reacting a bromide salt with a source of hypochlorous acid, the optimum mole ratio of chlorine to bromide is near 1. However, no information is provided as to how to maintain the ratio near the optimum in the recirculating water while chlorine and bromide salts are being fed simultaneously to the system as well as being lost from the system.